A conventional center drill 50 is disclosed in FIG. 12 and generally includes a cylindrical shank 51 which is sized to be clamped by the chuck of machine (not shown). Two bits 52 are connected both ends of the shank 51 and the size of the bits 52 are vary according to the size of the shank 51. If the size of the bit is 1.6 mm, the shank 51 will be 4.0 mm, the size of the bit is 4.0 mm, the shank 51 will be 10.0 mm, and the size of the bit is 10.0 mm, the shank 51 will be 25.0 mm. In other words, the larger the bit size is, the larger the shank will be. However, the shank 51 does not have any further function except being connected to the chuck of machine, so that most of the center drills are made by High Speed Steel (HSS) which is inexpensive.
The HSS cannot cut at high speed and the slow movement of the center drill means a longer processing time is required which increases the cost. In order to work rapidly, the center drill can be made by Tungsten Carbide and the center drill made of Tungsten Carbide is able to drill objects 5 to 20 times faster than the HSS made center drill. Nevertheless, the cost of the Tungsten Carbide is 10 times of that of HSS. The shank requires much more material than the bit and usually contributes most of the expense of the Tungsten Carbide.
The present invention intends to provide a flat center drill and the drill holding tool, wherein the flat center drill saves significant material and can be easily and precisely connected to the holding tool.